Discovering oil or natural gas, is directly related to its depth within the producing earth formation. That is to say, the shallower the reservoir below the earth's surface, the greater likelihood of its discovery and subsequent development.
Since shallow oil fields were thus first discovered and developed, they are now among the oldest of all presently producing fields. But they also represent a surprisingly large proportion of oil production in may regions, especially in the United States.
These fields are further characterized by relatively low production rates. In California, Texas and Oklahoma, for example, present average production rates are well below 100 barrels per day per well.
At the same time, many of these wells suffer from plugging problems associated with their age or environment, i.e., channels within the formation and/or perforations within the casing, become filled with particulate matter such as sand. There many be an additional requirement for mechanical pumps to lift the oil to the surface. Hence, many of these fields have reached a point in their economic lives, where the rates of producing wells are marginal when compared to the present worth of a barrel of oil.
Where the estimated oil-in-place has been fully depleted, nothing can be done. But where there is still a minimum volume of oil-in-place, a study of the present worth of oil might indicate that the field wells could be economically worked. For example, sufficient oil may justify: the use of methods to remove sanding problems; institution of low pumping rates; capping the wells and await future development; or selling the lease.
Heretofore, pumping assistance has been most prominently achieved by lowering a pump into the wellbore to a position below the surface of the pool of oil and water. The pump is then activated by a series of reciprocating rods positioned in the wellbore. At the earth's surface, a walking beam and counterweight at the earth's surface is usually used to initiate rod movement. Since the beam and weight resembles a horse's head, this pump type is called a "horse head pump".
In order to install the horse head pump, a large tower is required to be placed over the wellbore. After pumping has been started, the tower is removed, all at considerable expense. Additional problems associated with such pumping systems, aside from initial high costs, are as follows: (i) High maintenance costs due in part to failures due to "pump pounding" vibration, i.e., fluid level falls below a minimum at the downhole pump, (ii) Large energy needs to drive the downhole pump, (iii) High operating expense especially in those situations where downhole packers are used to avoid sanding problems, (iv) Noise, and (v) Low pumping efficiency in certain situations as where the oil is highly viscous.
In these circumstances, the prior art has recognized the need for a more economical means for lifting the oil and water. For example, in U.S. Pat. No. 4,552,220 for "OIL WELL EVACUATION SYSTEM", an endless belt is described to be driven from the earth's surface towards an anchoring unit at the bottom of the wellbore. The belt includes a series of connected linkages to which a plurality of open containers are appended. The containers dig into the accumulated matter near the bottom of the wellbore and then pass upwardly though the water and oil and thence to the earth's surface. After being inverted, the containers return to the wellbore and repeat the operation. But since the containers extend radially from the belt near the side wall of the wellbore, care must be exercised to choose situations where the wellbore is both straight and large enough to handle both the containers and belt. Also since the containers pass through the water first on the return trip up the wellbore, lifting is sequential: water is produced first followed by oil. Moreover, such design specifies that enlarged plastic shoulders on the containers be used to minimize the contact area. In addition, centering rollers are also recommended. Consequently, such a system is difficult to construct, expensive to operate and costly to maintain.
In U.S. Pat. No. 3,774,684 for "OIL MOP METHOD AND APPARATUS FOR PRODUCING AN OIL WELL", a rope-like belt is made from wound plastic fibers such as polypropylene for the purpose of selectively lifting oil, not water. Since the belt is also of circular cross section, the oil is scrapped off by squeezing the belt between a series of rollers. But only sectors of the belt are scrapped; the sides are unaffected. Hence use of such system can be inefficient in those applications where both oil and water are to be lifted simultaneously.
In U.S. Pat. No. 4.089,784 for "BELT TYPE OIL REMOVAL UNIT", a belt skimmer is described comprising an endless belt loop that can be submerged in the oil. The belt is driven by a rotary drum that has an irregular surface and a spring-biasing means to urge the belt against the drum in a positive manner. The oil adheres to the belt, is carried up to the earth's surface and then is cleaned by a scraper. But since the belt is formed of plastic materials that are hydrophobic, such system cannot be used effectively in those applications where both oil and water are to be lifted simultaneously or where it is desired to reduce the fluid level within the wellbore as rapidly as possible. Also a scraper is undesirable in many circumstances as where wear of the belt is a problem.